[PATCH 4/4] sched/fair: Return an idle cpu if one is found after a failed search for an idle core

From: Mel Gorman
Date: Tue Dec 08 2020 - 10:36:57 EST


select_idle_core is called when SMT is active and there is likely a free
core available. It may find idle CPUs but this information is simply
discarded and the scan starts over again with select_idle_cpu.

This patch caches information on idle CPUs found during the search for
a core and uses one if no core is found. This is a tradeoff. There may
be a slight impact when utilisation is low and an idle core can be
found quickly. It provides improvements as the number of busy CPUs
approaches 50% of the domain size when SMT is enabled.

With tbench on a 2-socket CascadeLake machine, 80 logical CPUs, HT enabled

5.10.0-rc6 5.10.0-rc6
schedstat idlecandidate
Hmean 1 500.06 ( 0.00%) 505.67 * 1.12%*
Hmean 2 975.90 ( 0.00%) 974.06 * -0.19%*
Hmean 4 1902.95 ( 0.00%) 1904.43 * 0.08%*
Hmean 8 3761.73 ( 0.00%) 3721.02 * -1.08%*
Hmean 16 6713.93 ( 0.00%) 6769.17 * 0.82%*
Hmean 32 10435.31 ( 0.00%) 10312.58 * -1.18%*
Hmean 64 12325.51 ( 0.00%) 13792.01 * 11.90%*
Hmean 128 21225.21 ( 0.00%) 20963.44 * -1.23%*
Hmean 256 20532.83 ( 0.00%) 20335.62 * -0.96%*
Hmean 320 20334.81 ( 0.00%) 20147.25 * -0.92%*

Note that there is a significant corner case. As the SMT scan may be
terminated early, not all CPUs have been visited and select_idle_cpu()
is still called for a full scan. This case is handled in the next
patch.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
kernel/sched/fair.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 413d895bbbf8..ed6f45832d97 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6066,6 +6066,7 @@ void __update_idle_core(struct rq *rq)
*/
static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int target)
{
+ int idle_candidate = -1;
struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
int core, cpu;

@@ -6085,6 +6086,11 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
idle = false;
break;
}
+
+ if (idle_candidate == -1 &&
+ cpumask_test_cpu(cpu, p->cpus_ptr)) {
+ idle_candidate = cpu;
+ }
}

if (idle)
@@ -6098,7 +6104,7 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
*/
set_idle_cores(target, 0);

- return -1;
+ return idle_candidate;
}

/*
--
2.26.2